The unit on cell cycle regulation uses Drosophila oogenesis as a model system to examine the developmental regulation of the cell cycle. Our current research focuses on two problems: the switch from the mitotic cycle to the meiotic cycle and the relationship between cell cycle regulation and oocyte differentiation. Animal oocytes undergo a highly conserved developmental arrest in prophase of meiosis I. Often this marks a period of rapid growth for the oocyte and is necessary to coordinate meiotic progression with the developmental events of oogenesis. In Drosophila a single oocyte develops within a 16-cell germline cyst. Throughout much of oogenesis the oocyte remains arrested in prophase of meiosis I. In contrast, its 15 mitotic sisters enter the endocycle and become polyploid in preparation for their role as nutritive nurse cells. How germline cysts establish and maintain these two independent cell cycles is unknown. We have demonstrated a role for the p21CIP/p27Kip1/p57Kip2 like cyclin-dependent kinase inhibitor (cki) dacapo (dap) in the maintenance of the prophase I meiotic arrest of the Drosophila oocyte. In Drosophila cyclinE-Cdk2 activity is required for entry into S phase. In the absence of Dap, the oocyte enters the endocycle and develops as a nurse cell. These studies have revealed a novel meiotic function for the Cip/Kip family of Cdk inhibitors. We are continuing to examine how Dap is spatially and temporally regulated during germline cyst formation and maturation. We have used a genetic screen, based on the FLP/FRT site-specific recombinase system, to identify additional genes that regulate the cell cycle during oogenesis. The FLP/FRT system allows one to examine small clones of mutant tissue in the background of a phenotypically wild-type animal. Using this technique we recently identified the 183B gene. Mutations in 183B result in the loss of the prophase I meiotic arrest of the oocyte. Antibodies generated against the 183B gene product indicate that it is one of the first proteins to be localized specifically to the oocyte nucleus during the differentiation of the ovarian cyst. Molecular analysis shows that 183B has limited homology to a family of retinoblastoma protein binding factors. We are currently continuing the phenotypic analysis of the 183B mutants. In addition, we are attempting to identify genes that interact with 183B using both genetic and biochemical approaches. In metazoans relatively little is known about what controls the switch from the mitotic to the meiotic cycle during gametogenesis. In our FLP:FRT screen we identified a mutant in which ovarian cysts undergo an extra mitotic division before entering the meiotic cycle. Molecular analysis of this mutant indicates that it is an antimorphic allele of the twine gene. twine encodes a germline specific form of the mitotic activator cdc25. We are currently using this mutant to examine how the timing of the germline cyst divisions influence entry into the meiotic cycle.